Calcium ions were microinjected, ionophoretically or by pressure, into droplets of the photoprotein aequorin. For pulses less than about 1 s in duration, the amplitude and rate of rise of the light emitted as a result of the combination of calcium with aequorin, were directly proportional to the ionophoretic current flowing through the Ca pipette. Increasing the concentration of aequorin led to a proportional increase in the amplitude of the light responses. Pressure injection of a known amount of Ca2+was used to estimate the transport number for Ca in 3 ionophoretic pipettes which gave a mean transport number of 0.11.